Assessing the Substrate and Diagnosing Coating Failure
CSLB divides the official C-33 Painting and Decorating examination into four sections: Planning and Estimating at about 28 percent, Substrate Repair and Surface Preparation at about 30 percent, Application of Paint, Wallcoverings, and Finishes at about 21 percent, and Safety at about 21 percent. Surface preparation is the largest single block, and it is also where nearly every real-world callback originates, so this guide gives it three chapters. This first one is about looking at a wall and knowing what you are seeing. The scope of the license itself is set by 16 CCR section 832.33, which describes a painting and decorating contractor as one who prepares surfaces by scraping, sandblasting, or other means and applies paints, papers, textures, fabrics, pigments, oils, varnishes, shellacs, stains, fillers, waxes, adhesives, and similar materials to the surfaces of structures for decorating, protecting, fireproofing, and waterproofing. Notice that preparation is written into the definition of the trade.
The Sequence That Everything Else Depends On
The basic order for a repaint is clean, scrape, sand and patch, then prime, and each step exists because the next one cannot succeed without it. Cleaning comes first because contamination defeats adhesion regardless of what you do afterward: paint applied over grease, oil, chalk, or dirt is bonded to the contaminant rather than to the substrate, and the contaminant is bonded to nothing. On a chalky exterior stucco wall, the very first action is to wash the surface to remove chalk, dirt, and loose material, because chalk is the powdered remains of a degraded binder and a coating applied over it peels in sheets carrying the chalk with it. Scraping removes what is already failing, and it is taken back to a sound edge, meaning an edge where the remaining film is firmly attached, then feathered by sanding so the transition between bare substrate and old paint does not telegraph through the new finish. Feathering is exactly that, sanding the edges of a spot repair smooth so there is no ridge. Sanding and patching follow, and after sanding, all dust must be removed, because dust on the surface prevents adhesion and leaves a gritty finish. Priming closes the sequence by sealing the surface and giving the finish coat a uniform base. Skipping a step does not save time; it moves the cost from your labor line to your warranty line.
Reading a Failed Film
Coating failures have distinct appearances, and the appearance tells you the cause, which tells you the repair. Blistering is the film lifting away from what is under it in bubbles, and the diagnosis is made by cutting one open and looking at the bottom. If bare substrate is visible at the bottom of the blister, moisture came from behind and pushed the entire coating system off the substrate, so the water source has to be found and corrected before anything is recoated. If the old paint is still in place at the bottom, only the bond between the new coat and the old film failed, which points to a preparation problem such as a glossy or dirty surface. Alligatoring is a coarse pattern of cracks resembling reptile hide that runs through the entire paint film, and it is caused by a thick, brittle buildup of coats that can no longer move; the film has to be removed rather than coated over. Fine hairline cracks running with the grain of an old oil-painted sash indicate an aged, brittle film that no longer flexes as the wood moves seasonally. Widespread peeling down to bare wood almost always means a preparation or moisture problem underneath rather than a bad batch of paint. Chalking is judged in the field by rubbing a dark cloth across the surface and looking at how much pigment transfers. Efflorescence, the white crystalline deposit on masonry and concrete, is soluble salt carried to the surface by water moving through the substrate, so it is a water problem before it is a paint problem.
Moisture, Alkalinity, and Adhesion Testing
Three measurements separate professional preparation from guessing. Moisture in wood is checked with a meter before painting, because wet wood traps moisture behind the film and blisters as it tries to escape. An exterior that was pressure-washed at four in the afternoon in cool, humid weather will very likely still hold moisture at eight the next morning, and coating it is how a fresh job blisters within a season. Concrete has its own tests. A calcium chloride test or a plastic-sheet test tells you whether a slab is dry enough to coat at the surface, while in-situ relative humidity probes set into the slab measure moisture inside the slab, which is what a specification wants before a floor coating goes down. Alkalinity is the second measurement. Fresh concrete, stucco, plaster, and masonry are strongly alkaline, and that alkalinity attacks many coatings, saponifying an alkyd film into a soft, soapy layer. That is why an alkyd enamel over four-day-old plaster is a real risk, why a stucco wall floated eight days ago should be allowed to cure or coated with an alkali-resistant primer, and why a painter checks surface pH before coating a two-month-old tilt-up wall. Adhesion of an existing coating is checked before recoating with a cross-hatch tape test through the film in accordance with ASTM D3359, which cuts a lattice into the coating, applies and removes tape, and rates how much film comes off.
Mildew, Stains, and Odors That Come Back
Some problems are not preparation problems at all; they are contamination that will migrate through whatever you put over it, and the answer is always a barrier rather than more finish coats. Mildew must be killed and washed off before repainting, not painted over, because it will grow through a fresh film. The quick field check for whether dark spotting on a shaded eave is mildew or accumulated dirt is to dab a drop of household bleach solution on a spot: mildew lightens quickly, dirt does not. Tannin bleed is the classic wood problem. Redwood and cedar contain water-soluble extractives that migrate through a waterborne film and appear as brown-tan discoloration, often on the sunniest wall, weeks after the job is finished. The prevention is a stain-blocking primer rated for tannin bleed applied before the topcoat, and the same logic covers knots and sap streaks in bare softwood, which are sealed with a stain-blocking or shellac primer. Water stains on a ceiling are handled by first repairing the leak, then drying the drywall, then spot-priming the stain with a stain-blocking primer, and only then coating the ceiling. Smoke odor after a fire and crayon or permanent marker that keeps ghosting through a repaint are both handled with a pigmented shellac-based sealer, which blocks materials that waterborne primers cannot hold back. Rust rings appearing over nail heads on a painted fascia are fixed by setting the nails, spot-priming the heads with a rust-inhibitive primer, and then filling.
Patching, Caulking, and Filling
Interior patching follows a fixed order. A small nail hole in drywall is filled with spackle, allowed to dry, sanded smooth, and then spot-primed, and the spot-priming is the step people skip. Every patch is more absorbent than the surrounding painted wall, so an unprimed patch drinks the finish coat and reads as a dull halo under raking light, even though the color is correct. Larger holes are built up in multiple thin coats of joint compound rather than one thick one, because thin coats dry evenly and are far less likely to crack and shrink. Where a repair must be filled and topcoated the same day, a setting-type joint compound cures chemically rather than by drying, which is what makes that schedule possible. On a lobby wall with floor-to-ceiling glass, where raking light will expose every joint, the honest preparation is a skim coat over the full wall to equalize texture and porosity before priming. Caulking is a sealing operation, not a cosmetic one: the purpose of caulking gaps around window trim is to seal the joint against water and air intrusion. There is one joint you deliberately leave alone. On horizontal lap siding, the bottom edge where each course laps the course below must stay open, because that gap is the drainage and drying path for the wall assembly, and sealing it traps water behind the siding. Exterior filler must be exterior-rated: a deep gouge in exterior trim filled with lightweight interior spackle will fail within a season, where an exterior-grade epoxy or polyester wood filler will not. A moving joint, such as an expansion joint between stucco panels, needs a high-movement sealant installed over backer rod with the correct joint profile, and the backer rod sets the sealant depth and keeps the bead bonded on two faces only so it can stretch.
Knowing When to Stop and Tell the Owner
Part of a painting contractor's professional duty is recognizing when the problem in front of you is not a painting problem. Hardboard lap siding that is swollen and soft along the bottom edges of several courses is water-damaged, and the correct sequence is to replace the swollen boards and correct the water source first, not to prime and paint over a substrate that is disintegrating. A stucco wall showing a stair-step crack through the mortar joints with a widening gap at a corner is displaying structural movement, and the honest recommendation is that the movement be evaluated before any coating work is scheduled, because an elastomeric coating over an active structural crack will simply tear. A deck coated with a solid-color acrylic stain that is peeling across roughly half its area is not a candidate for spot-scraping; the defensible recommendation is to strip the coating from the whole deck, because the remaining half will fail on the same schedule and the customer will have paid twice. An owner who wants well-bonded vinyl wallcovering painted rather than removed is asking for a finish that will eventually delaminate along with the covering; the sound recommendation is to remove the covering, wash off the adhesive, and repair the wall. In each of these cases, documenting the recommendation and the owner's decision in writing protects both parties.
Last updated: September 2026